1. Field of the Invention
The present invention generally relates to an improvement of an apparatus for producing a magnetic recording medium, and particularly relates to an apparatus for correcting curling of a magnetic recording medium composed of a ferromagnetic thin film formed on a flexible substrate.
2. Prior Art and Problems Thereof
A so-called thin film magnetic recording medium in which a ferromagnetic thin film is formed on a substrate through vacuum evaporation, ion plating, sputtering, or the like, has received much attention as a medium having high recording density. The practical applications of such a thin film magnetic recording medium has likewise been earnestly investigated. A flexible substrate mainly composed of a plastic base, for example, of polyethylene terephthalate, polyimide, polyamide, or the like, has been widely used as the substrate of the thin film magnetic recording medium because of their various advantages in practical use.
In the case where such a plastic flexible substrate is to be used for in the thin film medium, however, when a ferromagnetic thin film is formed on the substrate, for example, through vacuum evaporation, the surface of the substrate is heated to a considerably high temperature by radiant heat from an evaporation source, heat of evaporated atoms, and so on. As a result strong internal stress is generated in the ferromagnetic thin film causing the resulting thin film recording medium to curl severely. This curl remains even after the recording medium is slit to obtain tape-like media, resulting in the following problems.
First, proper contact between a magnetic head and the tape-like recording medium cannot be maintained so that faithful recording/reproducing cannot be performed.
Secondly, the tape-like recording medium cannot run smoothly in a tape deck.
Third, disturbance is caused during the winding of the tape-like recording medium so that the tape-like recording medium is damaged at its edge portions.
In order to eliminate the foregoing problems, a method has been proposed in which the substrate is made to undergo transformation in order to correct the curl of the magnetic recording medium. Such a conventional curl correcting method is disclosed in Japanese Patent Unexamined Publication No. 58-158027 in which the curl of a magnetic recording medium composed of a ferromagnetic thin film formed on a polyester film is corrected by making the magnetic recording medium come into contact with a heating member such as a heating roller or the like, with predetermined amount of tension at a predetermined temperature for predetermined time. In this method, however, cracks may be caused in the ferromagnetic thin film. These cracks are most probably the result of a thermal shock caused by the heating roller. The electromagnetic conversion characteristics are reduced or the amount of loss of the products is increased, as a result of these cracks.
Further, Japanese Patent Unexamined Publications Nos. 57-167136, 57-167137 and 57-167138 disclose a method for moderating the foregoing thermal shock, in which the temperature of a medium is raised at a predetermined rate to generate thermal shrinkage in the medium with a predetermined tension on the medium. Then the medium which has been subjected to heat shrinkage is cooled with a temperature descending gradient corresponding to the foregoing temperature ascending gradient. In this method, however, there have been problems in that it is necessary to correctly control the temperature along a predetermined temperature changing pattern, and the accommodation space and apparatus for performing heating and cooling is large scaled, the operation is complicated, and a relatively long time is required for treatment.